CriticalSectionLock: fixing missing include

Macro MBED_DEPRECATED_SINCE is defined in platform/mbed_toolchain.h which was not included.
If someone used member functions lock or unlock (which are prefixed with MBED_DEPRECATED_SINCE since some time), there would be a compile error instead of a warning.
Including mbed_toolchain.h fixes that.

Author: MikeDK

platform/CriticalSectionLock.h

@@ -19,6 +19,7 @@
 #define MBED_CRITICALSECTIONLOCK_H
 
 #include "platform/mbed_critical.h"
+#include "platform/mbed_toolchain.h"
 
 namespace mbed {